Improve display formatting of Stats and fix overflow error

Created by: femtobit

As observed in #459 (closed), the display issue in #460 also occurs when the error of mean is too small. With this PR, the decimal format is switched to scientific notation when the error is smaller than 1e-7, which makes the output more readable and also avoids this issue.

I've also added several other small changes to improve the output formatting since I was already making another PR here. (Using unicode such as σ² should be fine, we've already doing that with ± for a while and this output is only meant for human consumption anyway).